Generally, photographic materials containing a silver halide emulsion are subjected to various types of pressure in practical use. For example, when a conventional negative film for photography is taken up into a cartridge or is loaded into a camera, it will be bent or pulled from one exposure to the next. Also, other types of films (such as photographic materials for printing and radiographic materials for medical use such as X-ray films) are usually handled manually, they are frequently bent. Further, all types of photographic materials are subject to a great degree of pressure or stress when they are cut and worked on, such as in editing cinema films.
Thus, when various types of pressure or stress are applied to photographic materials, such will be exerted on the silver halide grains via the plastic film serving as a support or the gelatin (that is, a binder), supporting the silver halide grains, and the thus stressed part will often become fogged, desensitized or sensitized. When this happens, not only is the quality of the photographic image remarkably damaged, but also there could be risks which may lead to misdiagnosis in the case of reading radiographic materials or the like, which should be avoided at all costs. Such undesirable changes in photographic characteristics caused by these types of pressure or stress are reported in detail, for example, by K. B. Mather in Journal of Optical Society of America, 38, 1054 (1948), by P. Faelens and P. de Smet in Sei. et. Industry Photography 25, No. 5, 178 (1954), and by P. Faelens in Journal of Photographic Science, 2, 105 (1954).
Therefore, photographic materials whose photographic characteristics would not be influenced at all by the types of pressure and stress mentioned above have been keenly desired.
To improve such "pressure characteristics", it is known to decrease the silver halide/gelatin ratio of the silver halide emulsion and the method of adding an emulsifying agent to the photographic material but both of methods are insufficient. Specifically, for example, when the amount of gelatin is increased, the speed of the development treatment decreases, and when an emulsion is added, the mechanical strength of the emulsion layer decreases or the adaptability to high speed application becomes unfavorable.
On the other hand, it is also known that when a combination of gelatin and a synthetic polymer latex is used as a binder vehicle of a silver halide emulsion, the pressure characteristics can be prevented from worsening, without the above disadvantages. For example, in U.S. Pat. No. 3,632,342, it is disclosed that an acrylic acid type polymer latex is added to a silver halide emulsion layer for this purpose.
When a polymer latex is added to a silver halide emulsion, care must be taken to ensure the stability of the polymer latex to electrolytes. Since a polymer latex stabilized with an anionic surface active agent exhibits a quite low electrolyte stability, if the polymer latex is added directly to an emulsion, it together with the emulsion precipitates and the application becomes impossible. However, if a nonionic surface active agent is added to a latex, and then the latex is added to an emulsion, or if a latex prepared using, as an emulsifier, a surface active agent mixture of an anionic surface active agent and a nonionic surface active agent, which is added to an emulsion, the stability can be improved substantially. It has been reported in Kogyo Kagaku Zasshi, 64, 412 (1961) that the extent of this improvement is such that the greater the number of moles of ethylene oxide of a nonionic surface active agent added, the higher the improvement, as to effects of electrolyte stability observed. Further, the greater the amount of the nonionic surface active agent added, that is, the higher the concentration of the nonionic surface active agent, the higher the electrolyte stability.
Still further, in Japanese Patent Publication No. 54782/82, a method is disclosed of adding, to a coating composition, a polymer latex component impregnated with a nonionic surface active agent having ethylene oxide added thereto, as an antistatic agent. The anionic and/or nonionic surface active agents having added ethylene oxide disclosed in the Kogyo Kagaku publication and Japanese Patent Publication mentioned above, improve the electrolyte stability of the polymer latex on the one hand, while on the other hand, when the silver halide photographic material using the polymer latex is developed, undesirable development occurs in such a manner that portions where pressure is applied to the silver halide emulsion film (by, for example, a roller of an automatic developing machine) are fogged in the form of black spots on the resulting image (hereinafter referred to as black speckled marks).
Although a greater number of the moles of ethylene oxide added to a nonionic surface active agent and a greater added amount of such a nonionic surface active agent will result in improved electrolyte stability of the polymer latex, the amount of black speckled marks is increased accordingly. When conventional nonionic surface active agents are used, in particular, to be applied to radiographic materials, the sensitizing paper (screen) used in photographing is stained due to the contact of the sensitizing paper with the radiographic material. As a result, the radiographic material after development would have speckled or reticulate density unevenness (hereinafter referred to as screen stain), and the commercial value of the resulting product is damaged considerably.
Further, these surface active agents used as a stabilizer diffuse to the surface of the photographic material, thus rendering the antistatic surface properties undesirable.
It has not been disclosed that conventional nonionic surface active agents having ethylene oxide added thereto would have a range as to the amount of addition that would substantially satisfy both electrolyte stability as well as eliminating black speckled marks and screen stain.
An important point in the production of photographic materials by the addition of a polymer latex to a silver halide emulsion is the stability of the materials to mechanical pressure from outside sources during production. Measures to increase the stability to mechanical pressure are known, as disclosed in U.S. Pat. No. 3,287,289 and British Patents 1336061 and 1106717, i.e., methods wherein monomers having a carboxylic group or a sulfonic group are copolymerized so that the repelling force of the polymer latex surface may be advantageously used.
However, these techniques improve mechanical stability indeed, but an agglomerate is formed when the polymer latex is added to an emulsion, causing serious problems in the production step.